Concerning the Use of Sacraments
Or as my translation of Article XII has it, "not only to be signs by which people may recognize Christians outwardly". I generally use the 1 Corinthians quote following the Words of Institution, "As often as we eat this bread and drink this cup, we proclaim the Lord's death until he comes." So the reception of the sacrament is itself a form of preaching, though we may not generally think that way.
I imagine this is similar to the Orthodox assertion that the proper work of the laity in the liturgy is to say the "Amen!" Except here the Amen is signified by feet shuffling towards the front of the sanctuary, mouths agape as the body of Christ is received, heads dipped towards the chalice, and all kinds of other actions performed before and after (Amen, thanks be to God, thank you, oops...) I imagine we would more clearly understand reception of the sacrament as a form of testimony if we did it under duress- say, illegally, furtively, at night. The irony being this, that by doing it in that way, it would cease to become testimony, for the neighbors wouldn't see us loading up in cars in the bright daylight of Sunday morning to do our Christian duty.
Nevertheless, this is a valuable clarification, that a part of the Use of the Sacraments is itself proclamation, not simply the proclamation that occurs when the prayers and Words of Institution are spoken, but the proclamation of many bodies receiving the sacrament, a visible word receiving a visible word. A better understanding of this aspect of the sacrament would also go a long way towards helping us understand the sacrament as itself something that awakens and strengthens faith. Each week at the table, we are not doing something habitual. We do a new thing; we preach the gospel.